I think by giving his ultimate even a longer duration, it's just a huge buff to him.
Tryndamere is so difficult to fight due to his ultimate, not his passive despite what people believe. While his passive makes him really good in duels, there's also obvious windows of opportunity to abuse it. Tryndamere will either get ahead in laning phase and destroy the enemy team, or he will split push the entire game due to him being able to escape relatively easy without much of a challenge. He will slow you, dash away, and if he is about to die he can always ult and heal. Tryndamere in most cases however, can simply just duel whoever tries to stop him due to his ultimate. His abilities are actually pretty weak, his E doesn't do much damage, his W can be counterplayed very easily, and his Q isn't that much alone without wasting your entire passive which would significantly hurt his damage output.
Tryndamere needs a rework due to the fact that he will either get ahead in lane and stomp everybody, or he will splitpush until the game is over. Why? Because if he gets behind, he's pretty useless in teamfights anyways. However, he still can duel mostly everybody in the game with the exception of a few people and also can take turrets very fast and kill minions faster.
If you buff his ultimate, Tryndamere will become noticeably stronger, if you buff his other abilities, he'll become noticeably stronger. If you nerf them, he'll be useless. His entire identity is to either one-shot people going 20-0, or splitpush until the game is over. They can't just give him a new ultimate, considering his entire kit relies on his ultimate, and if you nerf his ultimate he'll be useless
A champion that revolves around toxic aspects of the game with a kit that relies almost solely on his ultimate is pretty bad. The champion has plenty of room for counterplay, but he's usually dominant in solo queue considering how easy it is for him to splitpush from the outter turret to your nexus.